Jump-starting a vehicle with a dead battery is a relatively risky activity, primarily for two reasons. Initially, as the owner's manuals of most current automobiles state, serious damage to automotive electrical systems can occur in the event of a short circuit, or in the case of a radical disparity between the voltage of the jumping battery and the nominal voltage of the receiving electrical system. Because most current automobiles rely on electronic memories for their operation, an error committed while jumping the battery can erase the memory and render the entire vehicle inoperative, regardless of whether its battery is recharged or not. Beyond that, there is a strong element of personal risk because batteries may explode during charging, causing chemical burns and blindness. While currently available automotive batteries are far more resistant to exploding than those of a decade ago owing to new battery formulations, incorporation of vent caps, etc., the risk still remains. This is particularly true in view of the fact that most people who jump-start vehicles do not know proper jumping procedures or do not take heed of the importance of proper procedures. Following are the "official" procedures to be followed while jump-starting a vehicle, as reflected by numerous vehicle owner's manuals and automotive clubs:
(1) Make sure the battery contents are not frozen. If they are frozen, do not attempt to jump the battery. PA1 (2) Bring the vehicle with the active battery near the vehicle with the dead battery and park it so that the vehicles are not touching. PA1 (3) Turn off all unnecessary accessories and lights on both vehicles, open their hoods, and be sure that their batteries have plenty of ventilation. PA1 (4) Start the vehicle with the active battery and let it run for at least two or three minutes at a fast idle to ensure the battery has a charge. PA1 (5) Put on protective eyewear. PA1 (6) Connect the positive booster cable clamp (generally red) to the positive terminal on the dead battery. PA1 (7) Connect the other positive booster cable clamp to the positive terminal on the active battery. PA1 (8) Connect the negative booster cable clamp (usually black) to the negative terminal of the good battery. PA1 (9) Connect the other negative booster cable clamp to a clean, unpainted area on the engine block or frame of the car with the dead battery. (Note that it is not necessary to connect the negative booster cable clamp to the negative terminal of the dead battery because the negative terminal is "grounded" to the engine block/vehicle frame.) PA1 (10) Now that the active and dead batteries are in electrical communication, the vehicle with the active battery should continue to run at high idle for five or more minutes to allow the dead battery to build a charge and to warm its electrolyte. PA1 (11) The vehicle with the formerly dead battery should be started and allowed to run at a high idle for a few minutes before disconnecting the booster cables. PA1 (12) The booster cables should be disconnected in an order reversed from that of steps (6)-(9).
The steps set out above are intended to minimize the possibility of short-circuiting and sparking in the vicinity of the batteries. Sparking can ignite hydrogen gas produced by the batteries, blowing the batteries apart and spraying acid and battery fragments. However, even if the aforementioned directions for jump-starting are followed precisely, there is still the potential for a spark. This is why current vehicle owner's manuals and automotive clubs first recommend that a professional service be called in the event of a dead battery, and that jump-starting by vehicle owners should only be performed when professional help is not available.
The primary cause of sparking (and explosion) is the accidental reversed connection of positive and negative jumper cables to a battery. It is generally easy to correctly connect booster cable clamps to the proper terminals of a new battery during the daytime since the positive terminals are marked with a plus sign and are generally colored red. However, as the batteries age they generally become covered with dirt and road grime, making the polarity of the terminals difficult to discern. Additionally, it can be difficult to make out the polarity of the terminals when little or no light is available.
The possibility of a spark is also enhanced by the fact that the positive and negative terminals of automotive batteries are already occupied by the clamps connecting the terminals to the vehicle's electronic system, thereby making space on the terminals scarce for the connection of jumper cable clamps. Because clamping area is small and scarce, there is an enhanced likelihood that a jumper cable clamp might come loose and slide or fall to contact areas having the opposite polarity. This likelihood could be decreased if the clamps of the vehicle's electrical system were removed prior to jumping, but the fact is that this is rarely done owing to lack of tools on hand and the time and inconvenience this would entail.